1. Field of the invention
The invention relates to method and apparatus for detecting laminar flow separation and reattachment and more particularly to a nonintrusive method and apparatus for simultaneously detecting laminar flow separation and reattachment by sensing the phase reversal of low frequency dynamic shear stress signals in the respective regions of separation and reattachment.
2. Description of the prior art
It is known that laminar flow separation is a major cause of the rapid flow deterioration of airfoil aerodynamic performance characteristics at low-chord Reynolds numbers, Rec, (Rec&lt;10.sup.6). At such low-chord Reynolds numbers, the flow boundary layer is generally stable and the presence of an adverse pressure gradient, as in the rear-pressure rise area, may result in a laminar separation with or without turbulent reattachment. In the absence of externally forced disturbances such as suction, blowing or turbulators, the separation can and frequently does extend back to the trailing edge of the airfoil resulting in a large pressure drag on the airfoil or wing surface.
Typically, a laminar separation bubble occurs when a turbulent reattachment takes place upstream of the airfoil trailing edge and downstream of the separation. The extent of the separation, the laminar region within the resulting bubble, depends on several factors, including the boundary-layers at separation, freestream turbulence, airfoil surface roughness, and other sources of disturbances that cause shear-layer transition. An understanding of the physical characteristics of laminar flow-separation bubbles is required to detect and ultimately control or eliminate undesirable laminar flow separation over air foils.
There are a number of known techniques for detecting laminar flow separation that make use of single point measurement instruments such as hot-wire anemometers and pitot tubes. Such instruments, because of the streamwise extent of laminar separation bubbles which can vary from a fraction of a percent to several tenths of the airfoil chord length and of their small height which is usually a small fraction of a percent chord, may be unsuitable as the internal sensors disturb the flow and other characteristics of the bubble detected leading to inaccurate measurements.
Flush mounted or buried hot-film or wire gages on or within the exposed airfoil surface for detecting transition and generally separation are also known. This type of sensor involves three spaced separate temperature responsive elements thus requiring a minimum spacing between the elements to avoid undesirable heat conduction on or through the substrate and airfoil surface. The heat conduction between the elements and through the substrate may be as great as the heat convection in the thin-shear stress layer at or near the wall formed within the bubble at the upstream point or line of separation.
Another technique, which is non intrusive, makes use of a laser velocimeter for sensing the velocity of moving particles within the laminar separation bubble. The accuracy of this instrument is dependent upon a sufficient number of particles in the laser control volume as they traverse through the separation bubble. Also low frequency shedding may occur affecting the accuracy of the resulting bubble measurements.
Typically, conventional hot film sensors may be arranged in arrays, the sensors being disposed in a line extending along the wing span transverse to the direction of air flow over the wing surface. Suitable electronic instrumentation is provided to interrogate each sensor in the array in seriation, thus detecting a line of laminar flow separation points that extends substantially along the span of the airfoil or wing but giving no indication of an exact point of separation and reattachment of a resulting separation causing the separation bubble along a line parallel to the flow of the airstream.